1. Field of the Invention
The present invention relates primarily to the field of calendering sheet material and more particularly to a system and method for improving the quality of a calendered paper sheet or the like.
2. Related Art
Paper quality is frequently measured by its printability and its runability. Printability of a paper sheet refers to the quality of image transfer to the sheet during the printing process and may be influenced by the sheet's finish (e.g., the degree of its gloss and/or smoothness). Runability, on the other hand, refers to the ability of the paper sheet to progress smoothly through the machinery of the printing process (i.e., paper having good runability will not tear or wrinkle during printing). The runability of the paper sheet is influenced by the cross-directional uniformity of sheet characteristics such as its caliper (thickness), strength and moisture content.
Paper production typically involves a calendering process which includes pressing paper material between calender rolls to obtain desired physical characteristics. Calendering paper can change its density, thickness, and surface characteristics (e.g., gloss and smoothness). Heat and moisture (usually in the form of steam) are frequently applied to paper being calendered to affect certain of its characteristics. Various grades of paper may thus be produced to suit various applications.
One of the parameters used in grading paper sheets is the finish of the sheet's surface. Typically, bulk paper is produced in a continuous sheet and wound in rolls having dimensions 12 to 36 feet in the cross-direction (i.e., across the width of the sheet). Cross-directional uniformity of surface finish of the bulk roll (i.e., across its width) is often desirable or necessary. For example, in the situation where the roll of paper is cut to size for making various paper products, the consistency of the finish of the individual paper items is dependent upon the uniformity of the finish of the original bulk paper roll.
The desired finish of a highly finished paper sheet is typically created by calendering the paper on a "supercalender" consisting of a stack of alternating hard polished steel rolls and soft or resilient cotton rolls. For example, a glossier finish may be created on the surface of paper by applying steam to the paper surface before or during supercalendering. The steam condenses on the paper surface and its surface fibers (or coating) are softened as the paper sheet travels through the calender stack. As the steam treated paper surface comes into contact with the calender rolls, it is smoothed by the pressing and rubbing actions of the polished steel roll and adjacent cooperating soft roll and a glossy finish is created. The degree of gloss created is dependent on the amount of moisture and heat and hence the amount of steam applied to the surface. A common problem encountered in creating a desired finish during calendering is the cross-directional non-uniformity of the finish obtained. Cross-directional variations in the contact load between adjacent rolls and other factors affect the uniformity of the finish profile created on the calendered sheet. To address this problem, steam application devices which independently vary the amount of steam directed at various cross-directional sections of the sheet may be provided on the calender stack. Unfortunately, adjustments made to the surface finish profile of the paper sheet during calendering using steam typically degrade the cross-directional uniformity of the moisture content of the finished sheet. For example, sectionalized steam application for creation of a uniform gloss profile typically creates a 0.5% moisture variation in the profile of 30 to 70 Lb/3300 Ft..sup.2 finished sheet. Supercalendered sheets typically contain only approximately 4% moisture. Accordingly, a 0.5% cross-directional variation can be quite significant.
A non-uniform moisture profile of the sheet, in turn, has a negative affect on the runability of the paper sheet. Specifically, variations in the moisture profile of the paper sheet may create areas of stress concentration. Such areas are prone to tearing during the printing process. A non-uniform moisture profile may also have a somewhat negative affect on the printability of the paper sheet and may cause the paper sheet to curl and wrinkle when it is removed from its bulk roll.
Steam treatment during calendering thus has "coupled" negative and positive affects on paper quality, i.e., an improvement in cross-directional finish uniformity may be coupled to a loss of runability and printability.